Explain these mfj259

[Truck]

If you have 1.2< swr but your ohms are still too far out of tolerance what is there left to do? Also, what length coax to the antenna ddo most youu guys use? i have went from 12f. to 18ft. in the past and everything went crazy!

[jessejamesdallas]

Originally Posted by Truck

If you have 1.2< swr but your ohms are still too far out of tolerance what is there left to do? Also, what length coax to the antenna ddo most youu guys use? i have went from 12f. to 18ft. in the past and everything went crazy!

Your ohms will come down the closer you get to 1.1 SWR on a MFJ...Once the MFJ reads 1.0 to 1.1, then you'll see the ohms closer to 50...If you can't get the SWR to read 1.0 to 1.1, you'll never get the ohms to read where you want them on a MFJ.

You can try moving the antenna to a different location, and also try different coax lengths to the antenna...If that don't work, try a different antenna.

What antenna are you trying to tune anyway?

[Truck]

Well, i have it tuned to a 1.1 on the freq. we run which is 27.6550 AM It is a Sirio P5000.

also i run a 4x12 and the antenna is on a magnet, and i ran a ground wire right off where the shield is grounded

['Doc]

As already explained, there are a couple of 'catches' to using an MFJ-259 to get accurate readings. One of those 'catches' is that the feed line connecting the '259 and antenna should be an electrical 1/2 wave long. That electrical 1/2 wave feed line has a characteristic of showing an impedance at one end that's the same as at the other end. That means that it's just like connecting that '259 directly to the antenna's input. Works the same for any whole number multiple of an electrical 1/2 wave length of feed line.
There are two ways of finding that electrical length of coax. Use the manufacturer's supplied VF to figure it on paper, which will get you a close 'ball-park' length. Or, use that '259 to find an electrical length. The instruction manual will tell you how to do that.
The MFJ-259 tells you three important things, SWR, the 'R' or radiation resistance of the antenna, and the 'X' or the reactance, both inductive and capacitive. Unfortunately, it can't tell the difference between those two types of reactance so you have to figure that out (not hard to do). Those three thing all interact, change one and you change the other two. Juggling those three things till you get an 'X' of zero means the antenna is resonant at that frequency (frequency is the fourth thing that '259 can tell you, just forgot to mention that, too lazy to go back and change it).
Resonance is the biggy! So is getting all the reactance out of that antenna because reactance doesn't do any good there. It can be good in other places, but not in the antenna it's self.
If you use the tuning knob on that '259 to find where 'X' is zero, you've found the resonance frequency of that antenna. The 'R' doesn't necessarily end up 50 ohms, in fact it very seldom does. For a typical mobile 1/4 wave vertical. that 'R' is lower than 50 ohms, probably around 20 to 35 ohms maybe, and is absolutely normal. You will see by the SWR meter in that '259 that the SWR will be something around 1.4:1 to around 2.5:1, depending on that 'R' number. If that antenna has zero reactance (X) it's resonant. If the resulting SWR is less than 1.5:1, it's as good as it can get, quit messing with it!

{Reactance (X) doesn't produce any signal, only resistance (R) does. Why you don't want any reactances in the antenna.}

That's the 'bare bones' way of tuning an antenna with a '259. The manual will tell you a bit about the 'why' it works this way. To really understand it, you have to learn about impedance, reactance, and how that all works. Not exactly the easiest thing to do, but certainly not that difficult. Here isn't the place for finding out how that works, it would take up too much space. (And I'm not the one who should try teaching it, I'm a lousy teacher!)
Have fun!
- 'Doc

[Old #7]

Question about the mfj ... I have been working on my 10k 17' shaft on my bronco .. now the best i can do is swr 1.5 X=8 r =56 . but if i use my workman . meter i can get it down to 1.1 ... and same with ratshack meter .. and the same goes for the radio meter .... is there a chance that my mfj .(new) needs to be calibrated.. been screwing with this for 3 days

['Doc]

I really doubt if your '259 is messed up. There honestly isn't much practical difference between the readings you're getting. Using only the 'R' from the '259 for calculation of SWR would give you about 1.12:1, and that's all an SWR meter can do. The 'X' of 8 isn't much (would be better if it were '0') so I just really wouldn't worry about it, it isn't worth worrying about.
- 'Doc

[Old #7]

Thanks Doc .. Im just getting every thing ready So when I hook up My pillbox . The radio will be happy .... I will leave it for now . maybe on the weekend . I will try again ..

[jessejamesdallas]

. I have been working on my 10k 17' shaft on my bronco .. now the best i can do is swr 1.5 X=8 r =56

A Predator 10K with a 17" shaft should tune allot lower on the MFJ than the numbers your showing...You should be able to get that antenna tuned on a MFJ to read 1.0 to 1.1 with X=0 on most if not all the channels...

Where do you have it mounted on the truck? If you have it in the middle of the roof, try moving it back near the back of the roof...If that's where it's at now, then you may need to move it to the center of the roof.

Believe it or not, with the 10K's this can make a big difference when it comes time to tune...

[Old #7]

james I did try both locations . front and back of cab . Its a full size bronco..with a fiberglass back .... coax length 18 ft ..... with a trimag mount . hope this helps you to help me figure this out ,,,

[pa629]

See what happens when you use a MFJ?? You introduce all sorts of extra info for you to worry about. Just check the SWR with a Rat Shack cheapo meter and send that MFJ off to a good home. Like mine, for instance. I'll feed it well and I have a large fenced in yard and two towers for it to play on.